The things they did and the places they conquered was amazing, and the way they did it even more so.

Coming from the Step outside China it almost seems like Temujin (Genghis Khan) united the Mongol tribes and conquered the Xi Xia kingdom in China just because. From there he kept expanding north and West, stretching all the way into Russia and parts of Europe before he died. After the death of Genghis Khan the Mongols would go on to conquer most of Europe, all of China, and pretty much everywhere in the known world at some point.

The way they went about this is the most impressive and terrible part. They liked to give ultimatums of surrender or perish, and would literally kill every single living thing in a city sometimes if they fought.
Genghis Khan either actually believed, or intentionally spread the idea that he was gods wrath upon the people he was conquering. They were known to behead people literally by the 50,000's and leave their bodies where they lay. It was said you could travel to cities the Mongols had sacked a generation later and still find bones spread all across the country side, and when they did burn the bodies you could see the fires from 100's of miles away.
The Mongols also liked to line up the women of cities that fought them and rape them before killing them, often in front of their families for added effect.

Cities that surrendered weren't much luckier, as the Mongols often spared them just to come back and sack them later, or ride into the city and sack it just because. They also liked to use the people who surrendered to them as their "front line" basically sacrificing them to their enemies to tire them out before sending in their real army. On more than one occasion the Mongols struck a deal for peace with an army in exchange for information, only to ride them down a few days later and kill them all anyway.

What is crazy though is besides all that they seemed like a decent people to live under. They practiced religious freedom and gave many areas autonomy to enact their own laws and rules.

Here is a map of how far they conquered

When Genghis Khan died in 1229 and passed the empire on to his sons he told them it was "god's will" that they take over the whole world, which they very nearly did, there is even a dynasty of Chinese Emperors called the Yuan Dynasty who are actually Mongols, who mostly didn't even speak Chinese.

If you believe half of what is written in "The Secret History of the Mongols", Genghis Khan never came close to losing a battle, and 90% of the battles the Mongols fought were absolute massacres, they were probably the greatest fighting force in the history of the world

The Mongols invaded Russia in winter and were successful

Genghis Khan appointed people based off of merit and not social class, many of his best generals were actually former enemies of his that he saw great things in and promoted.

Their women were trained as warriors as well, more than capable of protecting the home front and sometimes fighting in battles

The bow they rode with was 100lbs and they could fire 12 or more arrows/min

Basically the Mongols were awesome and completely terrible, Genghis Khan is like Hitler, Alexander the Great, and George Washington wrapped up into one guy